An Integrated Spectroscopy-Microkinetic Approach to Identifying Active-Site Distributions in Palladium Single-Atom...

Single-atom catalysts based on metal-doped carbonaceous materials are promising catalysts. However, the synthesis procedure dictates the local atomic environment, often resulting in a heterogeneous distribution of active sites with distinct properties. Here, we identify these active sites and quantify the ensemble effect of single sites on the activity and selectivity for the oxygen reduction reaction (ORR) of a palladium/nitrogen-doped-graphene single-atom (Pd-N-C) catalyst. By combining core-level spectroscopic data from literature with in-house DFT calculations and microkinetic modeling, we screened various local active site structures. A thermodynamic formation analysis complemented by XANES exclude edge and porous sites based on their poor agreement with experiment and energetics respectively. The remaining basal site distribution within the model's assumptions is best-fit quantified using absolute XPS calculations revealing a site distribution dominated by basal plane PdN₄C₁₀ (67%-51%) and nitrogen enriched PdN₄C₁₀ + 1N (49%-33%) active sites. The origin of stability for basal structures is geometrically rationalized by the trend in the number of stabilizing hexagonal rings. According to microkinetic ORR rate analysis, the onset potential for both basal active sites is governed by the initial hydrogenation step, whilst nitrogen enrichment increases activity but reduces H₂O₂ selectivity.